つぼみ菜 Another Japanese special vegetable. Tsubomi means bud. It’s the symbol of early Spring.
Well it seems the complete name is Hakata tsubomina as there exist another plant called tsubomina. Hakata is an alternative name of Fukuoka, the city of Kyushu.
This page gives some details.
They are the bud of a type of large size mustard green. The whole plant makes 3 to 5 kg but only the buds are gathered. The season is February~March.
I ate them in salad and in soup before. I think that most recipes for Brussel sprouts could be use. But they are particularly excellent in tempura.
The inside is white and creamy.